JP 2009-145273 A discloses a resolver as a rotation angle detector, which receives an excitation signal and outputs an amplitude-modulated signal by modulating an amplitude of the excitation signal in correspondence to a rotation angle of a rotary electric machine. The rotation angle of the rotary electric machine is calculated by performing synchronous detection on the amplitude-modulated signal outputted from the resolver. The rotary electric machine is controlled by turning on and off switching elements of a power converter electrically connected to the rotary electric machine in correspondence to the calculated rotation angle.
The amplitude-modulated signal outputted from the resolver is likely to be superimposed with noise, which has frequency components around an excitation frequency of the excitation signal. It is difficult to remove the noise from the amplitude-modulated signal. If the noise is not removed, the rotation angle cannot be calculated with high accuracy.